The presence of noise in a video signal can have a significant impact on picture quality. As the size of many video displays increase, the impact of noise in video signals tends to increase proportionally. Noise can be introduced into video signals from one or more different sources. Artifacts, for example, can originate from the imaging and recording equipment, from environmental circuitry, from transmission equipment, from communication channels, and from encoding and decoding equipment. The particular affects of noise can be different on different components of the video signal, including the luma and chroma components.
Various digital image processing techniques have been developed in an effort to reduce video noise. In one existing technique for removing noise from a video signal, an interframe low pass filter is constructed by making use of the fact that there is a high interframe correlation between image signals, but not much interframe correlation between noise. This technique efficiently removes noise from signal components in stationary images or slowly moving parts of images. In a post-processing scheme, a filter is introduced to remove coding noise in motion compensated interframe coding where the signals are assumed to be compressed.
Another example of video signal filtering is the decoding of composite television signals to separate luminance and chrominance information. A wide range of decoder circuits have been proposed to suit the differing demands posed by the numerous applications for decoders. These demands differ not only in the cost to performance relationship, but also in the behavior of the filter.
While filtering reduces noise in a video image, some current approaches can also reduce the resolution (e.g. sharpness) of the image, leading to imprecise edge transitions, thereby reducing apparent focus. These abrupt changes are typically oriented in a vertical or horizontal direction, such as an edge between a blue sky and a black building. Accordingly, an improved approach for reducing noise reduction for video signals is desired.